This application claims priority from German Application No. 199 15 891.6, filed Apr. 8, 1999, the disclosure of which is incorporated herein by reference in its entirety.
The invention relates to a transfer belt of a wet press with extended press gap for drying a paper web.
In wet presses of paper machines, a substantial portion of fluid contained in a fresh paper web is squeezed out between pressure rollers which form a press gap or, in the case of a so-called shoe press, between a press shoe and a counter roller. The paper web is usually guided by means of a continuous felt belt through the press gap in which the felt belt picks up fluid from the paper web and discharges it.
In a so-called tandem shoe press, dewatering is generally carried out between two press felts. In a new development, one of these press felts is replaced by a transfer belt, thus allowing dewatering output in the press gap (nip) to be improved and the gap between the last web press and the dry section of the paper machine (the so-called xe2x80x9cdry sectionxe2x80x9d) to be closed. A transfer belt should provide even pressure transfer within the press gap, offer good sheet delivery and not cause a substantial rehumidification of the paper web when running out of the press gap. Transfer belts used in practice are generally formed substantially of polyurethane and typically have a smooth, ground surface.
It has been found that the paper delivery properties of these polyurethane transfer belts may not be entirely satisfactory; furthermore, a paper web guided on them through the press gap typically has surfaces of unequal smoothness (xe2x80x9ctwo-sidednessxe2x80x9d); this two-sidedness is ordinarily viewed as a lack in quality in the case of graphic papers.
It is, therefore, an object of the invention to provide a transfer belt with improved application properties which, in particular, may have advantages in sheet delivery and surface texture of a produced paper web.
This and other objects are satisfied by the present invention, which encompasses the basic idea of replacing a conventional transfer belt having a smooth surface facing toward the paper web with one having a fibrous surface which substantially matches the surface texture of the press felt located on the other side of the paper web. Such a transfer belt can match the surface texture on both surfaces of the paper web and substantially dispose of the disadvantageous two-sidedness.
A transfer belt of the present invention can provide this feltlike, fibrous surface texture by a two or more component coating on a support belt, which is in particular woven or machined. The aforementioned multiple component coating is needled to the support belt and firmly joined thereto by thermo-mechanical techniques, (i.e. by a suitably chosen pressure and temperature application regime), such that the composite can withstand the high mechanical loads a transfer belt is subjected in the web press section of a paper machine. The aforementioned two or more component coating of the support which delivers the fibrous structured surface incorporates a relatively thick water-resistant low melting polymer coating which is firmly joined to the support belt and a thin fibrous surface coating of non-melting (relative to the polymer coating) and non-adhesive fibres which are themselves firmly joined to the polymer coating.
In some embodiments, the transfer belt is insulated on both sides with a polymer layer in order to prevent the xe2x80x9ccarrying alongxe2x80x9d of water on the underside of the belt.
Preferably low melting polyolefins, polyamides, polyesters, polyacrylate or polyvinyls are used for the polymer layer.
The low melting polymer layer is formed after the needling process or during the fusing process by melting the fibres of the polymer, typically at temperatures in the range between 100xc2x0 and 220xc2x0 C., preferably between 120xc2x0 and 150xc2x0 C. Through the application of pressure in the range between 5 kg/cm2 and 70 kg/cm2, the softening temperature of the polymer can be reduced and the compression of the belt increased.
The polymer layer is formed in that meltable or melt-adhesive components within a feltlike structure which is applied to the support belt are molten under pressure. This method of forming a polymer layer can provide a predetermined compressibility which substantially contributes to an even pressure transfer in the press gap and excellent delivery properties during transfer of the paper web into the dry part. The low thickness of the fibrous surface layer, which is arranged on the water-resistant polymer layer, can provide excellent delivery properties at the transfer of the paper web in the dry part and reduced rehumidification which disposes of a grave disadvantage of press felts relative to the present transfer belt.
In a preferred embodiment, the support belt has in particular a multi-layered or laminated structure of fine twists in all layers, both in the longitudinal and transverse direction. The fine fibres may be monofilaments with a wire thickness between 0.1 and 0.3 mm which can be multiply twisted, filament twists or mixed twists out of monofilaments and multifilaments. When the support fabric has a seam, coarser monofilaments with a wire thickness between 0.3 and 0.8 mm may be used.
In another embodiment, a fleece layer is provided on the side of the support belt facing away from the polymer layer which is firmly joined to the support belt and which can further improve the pressure transfer properties in the press gap. When applying a plurality of fabric layers for the support web, then these are preferably jointly needled, which produces in particular the fibre structure for forming the polymer layer simultaneously with joining the fabric layers.
The area of the transfer belt which following the thermo-mechanical finish forms the polymer layer is structured in particular of fibres with a proportion of melting or melt-adhesive fibres of at least 10%, though preferably in the region of between 25 and 100%. The polymer layer is in a particularly lasting design additionally reinforced by longitudinal and/or transverse threads additionally melted into the (needled) fibre structure.
The thickness of the polymer layer (or in the event two polymer layers are present, both polymer layers together) lies in the range between 20 and 90%, in particular between 60 and 90%, of the total thickness of the transfer belt. In contrast thereto, the mean thickness of the fibrous surface layer lies only in the region between 1 and 10% of the total thickness of the transfer belt. For practical reasons, the fibrous surface layer typically includes high temperature and friction resistant fibres, thus providing the opportunity for long life of both this surface layer and the transfer belt as a whole with constant texturing properties relative to the paper web. The fibrous surface layer is formed preferably out of fibrous material with a melting point that lies 50xc2x0 to 100xc2x0 C. higher than the melting point of the polymer layer out of e.g., polyester, polyamide or polycarbonate fibres, and/or with xe2x80x9cnon-meltingxe2x80x9d fibres such as PAC, aramid, Teflon or carbon fibres.
Due to high rigidity of a transfer belt, a design as a seamed belt or as a seamed felt fabric which is fully sealed when pulled in is particularly preferred.
Particularly suitable for an embodiment of the support belt are polyamide, polyester, and aramid as well as other fibres of high expansion resistance and great strength and flexibility. Polyamide, polyester, polyolefins such as polypropylene, and polyvinyls such as PVC fibres, as well as selected copolymers thereof, but also other readily available polymer fibres, can be used as needled fibres as well as for structuring the rear fleece. The aforementioned fibres have thermoplastic properties and are thus suitable for forming a dense, water-resistant polymer layer below the surface of the transfer belt which is facing the paper web. Precise setting of desired properties is carried out in a conventional manner by selection of base polymers of suitable structure, in particular chain length and degree of bonding, and, if appropriate, by an addition of softeners and other additives.
When forming the surface layer of thermoplastic fibres, it has to be observed in the production of the transfer belt that the thermo-mechanical treatment may be matched to the physico-chemical properties of the surface fibre material in such a maimer that substantially no fusing (with accompanying loss of texture) of the fibres takes place at the surface.
Advantages and practicalities of the invention are contained in the following description of preferred exemplary embodiments based on the figures.